Growing & Choosing Rhododendrons & Azaleas

By Kenneth Cox, Glendoick

RHODODENDRONS AND ACID SOIL

Rhododendrons need acid soil and they naturally grow on peaty soils in the Himalaya, Japan and China.

Soil acidity is measured according to the pH scale. Ideal is pH 4.5-6. Neutral is pH7 and higher than 7 alkaline.

Most soil in Scotland is naturally acidic.

Soil may have been artificially limed for farmland, growing vegetables etc. This washes out over 3-5 years. If liming has been done recently you can balance this by planting with peat. You can also use sulphate of ammonia to reduce soil pH: apply before planting as too much can burn leaves.

How do I know if I have acid soil?

Soil test kits are not always accurate: you may need to do several samples to get consistent results. If there are rhododendrons/ acid loving plants growing well nearby, your soil is acid. Best advice is to ask neighbouring gardeners.

SOIL PREPARATION & ORGANIC MATTER

Rhododendrons need an open soil mixture. Very heavy (clay) and very fine particles (silt) are not suitable.

To improve soil, making it more open (i.e containing air pockets) organic matter should be added: leafmould is the best. Alternatives are compost (own or bought), composted bark or conifer needles.

There is little point in spending money on rhododendrons and azalea if you are not prepared to do some soil preparation. Improve the soil in an area much bigger than the rootball so there is room to grow.

If drainage is good, then soil preparation need no more than 30cm (12in) deep.

Peat can be used to improve the soil but it is not neccessary if you have other substances to use. Peat is useful as it is sterile, acidic and helps hold moisture but it has little structure, no feed and no mulching value. Ericaceous compost which you can buy in garden centres is a mixture of peat, green waste and some fertiliser. For containers you are advised to add perlite to the mix to ensure good drainage.

CLAY SOIL

In heavy clay soil, a raised bed is best: 30-45cm deep on top of the clay soil. Make a soil-compost-bark-peat etc mix and plant into this. Glendoick Garden Centre Pagoda garden is an example of heavy clay soil with raised beds created on top of it.

Peat walls and raised beds in Glendoick Pagoda Garden

PLANTING DEPTH

Rhododendrons must not be planted too deep. The rootball should be just below the surface. If you bury the rootball, you may kill the plant. Do not put very thick layers of mulch over the rootball.

HOW TO PLANT

Ensure plant is well-watered (but allowed to drain) before planting. Mix some organic matter (see above) into the existing soil as a planting medium. Soil should be firmed up around the roots but do not stamp on the rootball; avoid compacting the soil. For bare rooted stock, October to early April is the best planting time. Container stock can be planted at any time but if planted May-August water through the first growing season.

SHADE

Rhododendrons dont grow flower well under greedy trees: tree roots will take most available moisture and lack of light creates straggly, shy-flowering plants.

The further north, the more light is required: in Cornwall you can grow in more shade than Scotland.

The worst trees are dense, greedy ones such as beech and sycamore. The roots of the tree will reach as far as the dripline (where the branches extend to). Ideally in summer you should see some sky overhead. If you can’t, you have a problem.

If you live in Scotland, ignore advice that advocate shade or part shade. Maximum light = maximum flowers.

Good trees to grow with rhododendrons: Japanese maples, flowering cherries, Sorbus, Crataegus (hawthorn), Eucryphia, conifers: pine, larch, spruce (Picea), firs (Abies), cedar. Plant dwarf rhododendrons and evergreen azaleas in full sun in Scotland. Deciduous azaleas, larger hybrids and species can take some shade.

WIND & SHELTER

Varieties with large leaves, early growth or which are on the tender side for your climate require shelter from wind, particularly from south westerlies and north easterlies. Options if shelter is poor:

Plant a shelter belt of vigorous trees and shrubs.

Use rokolene (spun plastic membrane) or similar material to help plants establish.

Plant hardy wind-tolerant rhododendron varieties on the windward side and less hardy varieties inside these.

RHODODENDRONS IN CONTAINERS OUTDOORS

Best choices for containers are dense, compact plants with good foliage. Those with unusual or coloured foliage are particularly good.

Evergreen azaleas, yak & R. williamsianum hybrids are best choices for containers outdoors. Ensure good drainage as rhododendrons hate waterlogged containers. Use peat/ericaceous compost (with John Innes added if you can get it) and add some perlite, grit or bark. Ensure there are plenty of drainage holes and that they don’t get blocked.

RHODODENDRONS IN CONTAINERS INDOORS

Tender scented varieties can be grown in greenhouse/conservatory and brought in to house in flower. Rhododendrons dislike central heating and seldom succeed as house plants so need to be grown in a cool greenhouse; outdoors in summer. Ensure pots have good drainage and keep on the dry side. Feed and repot when plant becomes very pot-bound. Do not over pot as Maddenia and Vireyas like to be a bit pot-bound. Indica (indoor) azaleas good indoors, best put outside in Summer.